What is natural selection?
Charles Darwin and Alfred Russel Wallace proposed that species change over time through natural selection. It needs only three conditions:
- Variation: individuals in a population differ in their traits.
- Inheritance: some of those differences are passed from parents to offspring through genes.
- Differential survival and reproduction: some variants survive and reproduce more than others in a given environment.
When all three are present, helpful inherited traits become more common over generations. The populationadapts to its environment. Evolution is this change in the genetic makeup of a population over time.
The peppered moth story
Peppered moths rest on tree trunks during the day, where birds hunt them by sight. The normal form (typica) is pale with dark speckles, well camouflaged against lichen-covered bark. The first dark form (carbonaria) was recorded near Manchester in 1848. By 1895, about 98% of moths there were dark, because soot had killed the lichens and blackened the trees. After clean-air laws in the 1950s, the bark grew lighter and the pale form became common again. Experiments, including a large study by Michael Majerus published in 2012, confirmed that birds eat more of the moths that stand out.
Genes, alleles and dominance
Moth colour is controlled mainly by one gene with two versions, or alleles: dark (D) and light (l). Each moth has two copies. The dark allele is dominant, so DD and Dl moths are dark and only ll moths are light. Because heterozygotes are dark, selection can act on the dark allele even when it is rare — which is why it spread so fast in polluted areas.
Mutation and genetic drift
- Mutation creates new alleles at random. It is the ultimate source of variation, but on its own it changes frequencies only slowly. Genetic research has shown the carbonaria mutation arose around 1819.
- Genetic drift is random change caused by chance — which individuals happen to reproduce. It matters most in small populations, where alleles can be lost or become fixed purely by luck.
Try this: set the mutation rate to 0, run the polluted forest for a few hundred generations, then switch back to a clean forest. If the light allele has been lost completely, nothing happens — natural selection can only act on variation that already exists. In real populations, mutation and moths migrating in from other areas keep rare alleles from disappearing, which is why the light form could make its comeback.
The model’s equations
If q is the frequency of the dark allele, the genotype frequencies before selection are:
DD = q² Dl = 2q(1 − q) ll = (1 − q)²Each phenotype survives with probability s, which falls as its contrast with the bark rises. The dark allele frequency among the survivors is:
q′ = (q²·s_dark + q(1 − q)·s_dark) / (s_dark·(q² + 2q(1 − q)) + s_light·(1 − q)²)The simulator then applies mutation and draws the next generation at random from 2N gene copies to include drift.
Key terms
- Allele
- One version of a gene.
- Allele frequency
- The proportion of all gene copies in a population that are a particular allele.
- Phenotype and genotype
- An organism’s observable traits, and the alleles it carries.
- Fitness
- Relative ability to survive and reproduce in a particular environment.
- Adaptation
- An inherited trait that increases fitness in a particular environment.
Real-world examples
- Antibiotic resistance: bacteria with resistance mutations survive antibiotic treatment and multiply — a major concern in medicine.
- Pesticide resistance in insects and herbicide resistance in weeds evolve the same way.
- Darwin’s finches: on the Galápagos, beak size shifted within a few generations after droughts changed the available seeds (studied by Peter and Rosemary Grant).
- Lactase persistence: in human populations with a long history of dairy farming, the ability to digest milk as adults became common.
Common misconceptions
- “Organisms evolve because they need to.” Mutations are random; selection only favours variants that already exist.
- “Individuals evolve.” Populations evolve; individuals only survive or don’t.
- “Survival of the fittest means the strongest.” Fitness means reproductive success in a specific environment — sometimes the best-camouflaged, not the strongest.
- “Evolution is always progress.” Selection tracks the current environment, which can change direction, as it did for the moths.
What the model simplifies
The model uses a single gene with two alleles, random mating, non-overlapping generations and a simple link between camouflage and survival. Real moths also migrate between areas, rest in varied places, and have an intermediate “insularia” form. The selection strength here is adjustable for teaching; historical estimates suggest dark moths had roughly a 10–30% survival advantage in polluted areas.
Key takeaways
- Natural selection requires variation, inheritance and differences in survival or reproduction.
- Allele frequencies change across generations; individuals do not evolve.
- Which trait is favoured depends on the environment, and can reverse when the environment changes.
- Mutation supplies variation, and drift causes random change, especially in small populations.